(1) Field of the Invention
The present invention relates to a control apparatus and method for an optical amplifier, an optical amplifier, an optical transmission apparatus, an individual band gain equalizer, a wavelength multiplexing transmission apparatus, an optical amplifier and a wavelength multiplexing transmission system using the same equalizer, and more particularly to a technique suitable for use in a WDM (Wavelength Division Multiplex) optical transmission system which lengthens the transmission distance by disposing optical fiber amplifiers, represented by an Erbium-doped fiber amplifier (EDFA), in multi-stage system.
(2) Description of the Related Art
In recent years, as one example of a WDM optical transmission system, there has been noted a metro core system capable of adding/dropping an optical signal with an arbitrary wavelength at arbitrary nodes making a connection between local base cities. FIG. 16 is a block diagram showing an example of a configuration of a metro core system. In a system shown in FIG. 16, a plurality of OADM (Optical Add-Drop Multiplexer) nodes 100 are connected through transmission lines (optical fibers) 400 into a ring configuration so that, at each of the OADM nodes 100, a signal with an arbitrary wavelength (channel) is added to the transmission line 400 and, of WDM signals transmitted through the transmission line 400, light with an arbitrary wavelength is dropped therefrom. Moreover, optical amplifiers (preamplifier 200 and post amplifier 300) are properly placed at the former and latter stages of each of the OADM nodes 100, thereby compensating for the loss in light level between the OADM nodes (each of which will hereinafter be equally referred to simply as a “node”) 100 for lengthening the transmission distance.
In addition, in the system which adds/drops a signal at an arbitrary node 100 as mentioned above, since the number of signal wavelengths (hereinafter referred to equally as the “number of transmission wavelengths”) to be transmitted in the system (transmission lines 400) varies dynamically, for maintaining the output signal power for each wavelength (channel) to a constant value with respect to this variation of number of wavelengths (keeping the flatness of gain with respect to wavelength), an AGC amplifier having an automatic gain control (AGC) function is commonly used for the above-mentioned amplifiers 200 and 300.
In this case, for example, as shown in FIG. 17, the AGC amplifier is arranged such that portions of input/output signals of an optical amplifier (EDFA) 200 (300) are branched through the use of optical branch means 501 and 502 such as optical couplers so as to monitor the powers thereof (i.e., input/output signal powers of the optical amplifier 200 (300)) by PDs 601 and 602, and an automatic gain control unit 700 controls the pump power of the EDFA 200 (300) so that the power ratio therebetween becomes constant.
Meanwhile, in such a system, it is considered that, for example, as shown in FIG. 18(A), many (for example, 39 wavelengths) optical signals are added from one node 100 (100A) and a different one-wavelength optical signal is added from the next node 100 (100B). In this situation, for example, as shown in FIG. 18(B), in a case in which a trouble such as dynamic reconstruction of an optical transmission path, man-made mistake, fiber disconnection and connector removal occurs between the nodes 100A and 100B, only the signal added at the node 100B remains (that is, the number of transmission wavelengths varies suddenly. At this time, for example, as shown in FIG. 19(A), there arises a phenomenon that the power level of the residual light at the light reception end varies.
For example, as shown in FIG. 25, the above-mentioned “signal reception end” signifies an optical receiver 101 having an optical-electrical conversion function (O/E) to receive dropped optical signal for converting it into an electric signal, and this also applies to the following description. Moreover, the “signal transmission end” signifies an optical transmitter 102 having an electrical-optical conversion function (E/O) to transmit a transmission signal (electric signal) with added optical signal with a given wavelength.
For example, as shown in FIG. 19(B), the above-mentioned optical power variation can depend mainly upon three factors: (1) spectral hole burning (SHB), (2) gain (wavelength) deviation and (3) stimulated Raman scattering (SRS) effects, which will be described hereinbelow.
(1) SHB
The SHB producing the first factor is a phenomenon arising in the optical amplifier 200 (300) and shows a characteristic that the shorter-wavelength side light power lowers. That is, for example as shown in FIG. 20, when the optical amplifier 200 (300) amplifies light with one wavelength (for example 1538 nm) in the C band (1530 to 1565 nm), as a phenomenon, the EDFA gain lowers in the vicinity of that signal wavelength (which is referred to as main hole) and the EDFA gain also lowers in the vicinity of 1530 nm (which is referred to as second hole).
Moreover, in the C band, the main hole becomes deeper toward the shorter-wavelength side (gain decreasing quantity becomes larger) and both the main hole and second hole become deeper as the optical signal input power increases. Since this SHB is averaged in a state where multi-wavelength signal is inputted, it does not show a great effect. On the other hand, the effect thereof increases with a decrease in number of input wavelengths. For this reason, in a case in which only a signal with one wavelength remains due to the occurrence of a trouble between the nodes 100A and 100B as mentioned above, as shown in the column (1) of FIG. 19(B) and in FIG. 21, the gain of the optical amplifier 200 (300) decreases as the residual signal shifts toward the shorter-wavelength side, which causes a phenomenon that the output signal power also lowers (−ΔP). That is, the fluctuation of the gain due to the SHB varies in accordance with the number of signal wavelengths and the allocation thereof. The detail of the SHB is written in the non-patent documents 1 to 3 mentioned later.
(2) Gain Deviation
The gain (wavelength) deviation producing the second factor is also a phenomenon occurring in the optical amplifier 200 (300). That is, as mentioned above, the optical amplifier 200 (300) is made to maintain the average gain of the signal to a constant value (AGC) and, when a wavelength showing a deviation remains, it operates to adjust the gain of that signal to a target gain, which makes a variation (in this case, +ΔP) in output signal power of the residual optical signal, for example, as shown in the column (2) of FIG. 19(B). In other words, the operating point of the optical amplifier 200 (300) varies in accordance with the number of signal wavelengths and the arrangement thereof, which causes a variation in gain spectrum.
(3) SRS Effect
The SRS effect producing the third factor is a phenomenon occurring in the transmission line 400. An optical amplifier utilizing this SRS effect is a Raman amplifier. For example, as shown in FIG. 22, as a characteristic, the SRS of a common single-mode fiber has a gain peak on the lower frequency side by approximately 13 THz from the pump wavelength (when the pump wavelength is in the vicinity of 1400 nm, longer-wavelength side by approximately 100 nm) and, hence, the selection of the pump wavelength enables the optical signal amplification in an arbitrary wavelength band. However, as shown in FIG. 22, the amplification at one-point wavelength is not feasible, and since the amplification (gain) characteristic has a range in some degree with respect to wavelength, the amplification phenomenon arises even in the vicinity of the pump wavelength.
That is, when a WDM optical signal is transmitted through the transmission line 400, the shorter-wavelength side light power becomes pump power, thus amplifying the longer-wavelength side signal. In consequence, as shown in FIG. 23, there arises a phenomenon that the signal power increases toward the longer-wavelength side. Therefore, in a case in which only a signal with one wavelength remains due to the occurrence of a trouble between the nodes 100A and 100B as mentioned above, as shown in the column (3) of FIG. 19 and in FIG. 21 (B), as the residual signal shifts toward the longer-wavelength side, it is more difficult to take the power from the shorter-wavelength side, which causes the power (gain) reduction (−ΔP). That is, the SRS effect varies in accordance with the number of signal wavelengths and the allocation thereof.
Thus, when the number of wavelengths of WDM signal transmitted through the transmission line 400 varies largely, mainly depending upon the three factors of SHB, gain deviation and SRS, the output power of the residual signal (residual channel) varies. Even if the variation quantity per optical amplifier or transmission line is not very large, in the case of a system in which the optical amplifiers 200 and 300 made to carry out the AGC are provided in a multi-stage configuration, there arises the accumulation of the output signal power variations (ΔP) of the respective channels occurring the respective optical amplifiers 200, 300 and transmission lines 400.
In the case of a conventional optical transmission system in which the transmission distance is short and the number of optical amplifiers to be disposed in a multi-stage configuration is small, this variation is little and no problem arises. However, when the number of stages of optical amplifiers increases due to the lengthening of the transmission distance of the system in the future, for example, as shown in the left side of FIG. 24, the optical signal power at the signal reception end becomes out of a reception allowable range, which can produce transmission errors.
It is considered that the accumulation of the power variations is preventable by carrying out the automatic level control (ALC) at high speed at the occurrence of variation of the number of wavelengths, for example, as shown in the right side of FIG. 24. In this case, the ALC is usually made to monitor the output light power of the optical amplifier 200 (300) through the use of PD or the like for controlling (feedback-controlling) the pump power to the optical amplifier 200 (300) so that the monitored value reaches a target output signal power Pout which is a target output signal power Ptarget [dBm/ch] per channel×the number of inputted signal wavelengths.
Thus, for realizing the ALC of the optical amplifier 200 (300), the information on the number of inputted signal wavelengths to be inputted to the optical amplifier 200 (300) becomes necessary. However, in the case of receiving the information on the number of wavelengths from an optical service channel (OSC) or a network management system (NMS), it takes much time and cannot cope with the transient variation, for example, immediately after the occurrence of a trouble [for example, as indicated by meshing in FIG. 19(A), a variation in a period until the OADM node 100 shows the level compensation function after the occurrence of variation in the number of wavelengths].
For this reason, there have been proposed some techniques (methods) of calculating the number of wavelengths in the interior of a node.
(a) One technique is a method of, on the condition that the optical powers of the respective wavelengths to be inputted to an optical amplifier are even (inputted optical power per wavelength is known in advance), monitoring the total power of the inputted light to the optical amplifier to calculate the number of transmission wavelengths by dividing the monitored value by a specified inputted optical power per channel.
(b) As proposed in the following patent documents 1 and 2, another technique is a method of inputting light, branched in a manner such that a portion of signal inputted to an optical amplifier is used as monitor light, to a wavelength demultiplexer (DEMUX) to demultiplex it according to wavelength for counting the number of transmission wavelengths. Concretely, in the technique disclosed in the patent document 1, the inputted signal to the optical amplifier is monitored according to wavelength and the attenuation quantity of a variable optical attenuator provided at an output of the optical amplifier is adjusted in accordance with the monitored value and a variation in the number of wavelengths for controlling the output light power collectively. On the other hand, according to the technique disclosed in the patent document 2, in an optical amplifier in which optical amplification fibers such as EDF are connected in a multi-stage configuration, the pump power to each optical amplification fiber and the attenuation quantity of a variable optical attenuator provided between the stages of the respective optical amplification fibers are adjusted on the basis of the signal power and the number of wavelengths, detected from the inputted light to the former-stage optical amplification fiber, and the light power detected from the output light of the latter-stage optical amplification fiber, thereby controlling the gain of the entire optical amplifier and the gain spectrum.
It is desirable that the above-mentioned transient variation of the output power of an optical amplifier is compensated for (undergoes the flattening processing) as quickly as possible (for example, on the order of microsecond). As candidates for a technique of compensating for such an output power variation, there are, for example, (c) a configuration in which WDM output signal is demultiplexed according to wavelength and the optical power of each wavelength is adjusted through the use of a variable optical attenuator (VOA) for each wavelength and then multiplexed, (d) a dynamic gain equalizer (DGEQ), and other techniques. The DGEQ is a device designed to perform the loss adjustment for each wavelength of the WDM signal and capable of compensating for the gain deviation.
In addition, as a technique about the gain equalization, there are techniques disclosed in the following patent documents 3 and 4. The technique disclosed in the patent document uses an optical circulator, an optical reflector, a variable optical attenuator and a WDM coupler for carrying out the gain equalization according to a plurality of signals (wavelengths) demultiplexed by the WDM coupler. Moreover, the technique disclosed in the patent document 4 is related to variable gain flattening unit including a plurality of long-period gratings arrangement and an adjustment unit (piezo converter and piezo control circuit) for adjusting the attenuation factor for each grating.
[Patent Document 1] Japanese Patent Laid-Open No. 2001-168841
[Patent Document 2] Japanese Patent Laid-Open No. 2003-258348
[Patent Document 3] Japanese Patent Laid-Open No. HEI 10-173597
[Patent Document 4] Japanese Patent Laid-Open No. HEI 11-337750
[Non-Patent Document 1] Masato NISHIHARA, et. al., “Characterization and new numerical model of spectral hole burning in broadband erbium-doped fiber amplifier”, 2003 Optical Society of America.
[Non-Patent Document 2] Masato NISHIHARA, et. al., “Impact of spectral hole burning in multi-channel amplification of EDFA”, 2004 Optical Society of America.
[Non-Patent Document 3] Maxim Bolshtyansky, “Spectral Hole Burning in Erbium-Doped Fiber Amplifiers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 4, APRIL 2003.
However, with respect to the technique (method) of calculating the number of wavelengths in a node, there is a problem which arises with the method described above in (a) in that, when the power of each channel to be inputted to an optical amplifier varies, there is a possibility of calculating the number of wavelengths in error. Moreover, the method described above in (b) creates a problem of high cost and size increase. In the case of the technique disclosed in the patent document 1, since the loss is large in the wavelength demultiplexer (DEMUX), there is a need to increase the input branch ratio, which causes the deterioration of the noise characteristic (NF) of an optical amplifier, and in the case of the technique disclosed in the patent document 2, there is a problem in that the high-speed response becomes difficult.
Moreover, with respect to the equalization technology, in the case of the technique described above in (c) and the technique disclosed in the aforesaid patent document, since the received WDM signal is demultiplexed according to wavelength and the adjustment of the optical power is made according to wavelength through the use of the variable optical attenuator for each wavelength, the apparatus scale becomes larger to increase the cost. In particular, if a high-speed operating VOA is used for obtaining a high-speed response characteristic, this VOA is expensive and, when the VOAs corresponding in number to wavelengths are prepared, a further increase in cost occurs.
Still moreover, the dynamic gain equalizer described above in (d) creates problems in that the response speed is as relatively low as approximately 30 ms and the cost thereof stands at several millions yen and the adding loss is large (approximately 5 dB). Accordingly, the introduction into the system is impractical. Yet moreover, in the case of the technique disclosed in the aforesaid patent document 4, although, in a manner such that the piezo control circuit controls the piezo converter to change the pressure to be applied to the gratings, the characteristics of a plurality of grating can individually be changed so as to vary the attenuation factor of light passing through the gratings, the response speed is low because the change of pressure to the gratings falls under physical control.